1. Field of the Invention
This invention relates to fluid responsive switches such as flow switches and liquid level switches; and more particularly, relates to a metal to elastomer seal for a pivot arm used in a fluid responsive switch.
2. Background Information
Fluid responsive switches are generally used in fluid flow conduits to monitor and control the rate of flow or in containers, to monitor liquid levels. These devices include a housing, having a connecting body mounted on a nipple on the fluid conduit. The fluid responsive switch includes a pivot arm having one end positioned to respond to the flow of fluid in a conduit or the liquid level in a container. The other end of the pivot arm extends into the flow switch housing. The pivot arm is pivotally mounted in the fluid responsive switch body by a pivot pin and a elastomer seal that allows the arm to pivot according to the force applied by the fluid flowing in a fluid conduit or the liquid level in a container. The pivot arm is constructed to operate a microswitch in response to the fluid flow or liquid level.
The elastomer seal around the pivot arm, where it is mounted in the connecting body of the fluid responsive switch, has to be sufficiently resilient to allow the pivot arm to easily pivot while at the same time, maintain a fluid tight seal. Various designs have been developed to provide this function. For example, U.S. Pat. No. 4,074,097 discloses the use of a rolling diaphragm held in place by a clamping plate in a recess of the mounting body of a flow switch. The periphery of the rolling diaphragm is sealed by the clamping plate in the recess in the mounting body. The arm may easily pivot with very little resistance by the rolling diaphragm and yet the necessary fluid tight seal is maintained.
This design has been improved by the use of a molded resilient seal around the pivot arm and the pivot pin for mounting the pivot arm. This design employs an elastomer to metal bond to separate the fluid flowing in the conduit from the housing of the fluid responsive switch. However, bonding the elastomer to the metal surface of the pivot arm has been a difficult process to perform reliably since it involves a multitude of operations such as surface preparation, primer selection, primer application, primer baking, elastomer molding operation, etc. Each operation requires a specific temperature, pressure and time cycles. It is not uncommon to produce the same molded seal on the pivot arm using the same molding materials and processes, and yet suddenly start having a large, unacceptable rejection rate. The metal to rubber interface is almost always the source of the problem.
Many methods have been tried to produce a reliable metal to rubber seal on the pivot arm. These methods involve parameter adjustments that includes roughening the metal surface (i.e., sanding, filing and sandblasting). Roughening the surface is an old trick that is sufficient for use on the post of a toggle switch such as that shown in U.S. Pat. No. 3,898,397 or on the terminals mounted in an electrical housing such as that shown in U.S. Pat. No. 4,454,398. However, this method is not entirely satisfactory for a seal around a shaft subject to movement and is in contact with a corrosive fluid flowing through a conduit. A more dynamic, durable seal is needed in such environments. A solution to the problem of bonding an elastomer or rubber to the metal surface of a movable pivot arm in a corrosive environment would be advantageous.